Bioluminescent protease assay

ABSTRACT

A sensitive bioluminescent assay to detect proteases including caspases, trypsin and tryptase is provided.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims the benefit of the filing date of U.S.application Ser. No. 60/353,158, filed Feb. 1, 2002, under 35 U.S.C. §119(e). The disclosure of U.S. application Ser. No. 60/353,158 isincorporated by reference herein.

BACKGROUND OF THE INVENTION

Proteases constitute a large and important group of enzymes involved indiverse physiological processes such as blood coagulation, inflammation,reproduction, fibrinolysis, and the immune response. Numerous diseasestates are caused by, and can be characterized by, the alterations inthe activity of specific proteases and their inhibitors. The ability tomeasure these proteases in research or clinically is significant to theinvestigation, treatment and management of disease states. For example,caspases 3 and 7 are members of the cysteine aspartyl-specific protease(also known as the aspartate specific-cysteine protease, “ASCP”) familyand play key effector roles in apoptosis in mammalian cells (Thomberryet al., 1992; Nicholson et al., 1995; Tewari et al., 1995; andFemandes-Alnemri et al., 1996).

Proteases, however, are not easy to assay with their naturally occurringsubstrates. Moreover, many currently available synthetic substrates areexpensive, insensitive, and nonselective. Furthermore, the use of highconcentrations of the target protease, with either the naturallyoccurring substrate or a synthetic substrate, may be required for theassay, which may result in the self destruction of the protease.

Numerous chromogenic and fluorogenic substrates have been used tomeasure proteases (Monsees et al., 1994; Monsees et al., 1995) andmodified luciferins have provided alternatives to fluorescent indicators(U.S. Pat. Nos. 5,035,999 and 5,098,828). Methods for using modifiedluciferins with a recognition site for a hydrolase as a pro-substratewere first described by Miska and Geiger (1989). These heterogenousassays were conducted by incubating the modified luciferin with ahydrolase for a specified period of time, then transferring an aliquotof the mixture to a solution containing luciferase. Masuda-Nishimura etal. (2000) reported the use of a single tube (homogenous) assay whichemployed a galactosidase substrate-modified luciferin. Anon-heterogeneous luminescent protease assay has not yet been shown.

While luminescent assays are commonly known for their sensitivity, theirperformance relative to fluorescent assays is difficult to predict dueto fundamental differences in assay formats. Specifically, enzyme-linkedluminescence assays yield light coupled to the instantaneous rate ofcatalysis. In contrast, enzyme-linked fluorescence assays yield lightbased on the cumulative catalytic activity measured over a period oftime (a so-called “endpoint” assay based upon accumulation offluorophore). By integrating the catalytic activity over a period oftime that can extend from hours to days, the light signal from afluorescent assay can be greatly increased. Similar integration oversuch long periods is not practical for luminescent assays.

Thus, what is needed is a method to monitor protease activity that is arapid, single-tube, homogeneous, sensitive assay.

SUMMARY OF THE INVENTION

The invention provides a sensitive luminescent method to detect aprotease, e.g., a caspase, trypsin or tryptase. For instance, theinvention provides a luminescent assay method to detect one or morecaspases. The method comprises contacting a sample suspected of havingone or more caspases with a mixture comprising beetle luciferase and anamino-modified beetle aminoluciferin or a carboxy-terminal protectedderivative thereof, wherein the amino group of aminoluciferin or thederivative thereof is modified so as to covalently link a substrate forthe caspase via a peptide bond to aminoluciferin or the carboxy-terminalprotected derivative thereof. If the sample comprises a caspase having arecognition site in the substrate, the substrate is cleaved at thepeptide bond that links the substrate to aminoluciferin, yieldingaminoluciferin, a substrate for the luciferase, in the mixture.Luminescence is then detected. The method further comprises correlatingluminescence with protease concentration or activity, i.e., increasedluminescence correlates with increased protease concentration oractivity. Preferably, the luminescent assay is more sensitive than acorresponding assay with a conjugate comprising a fluorophore covalentlylinked via an amide bond to at least one substrate molecule or afunctional equivalent thereof. Thus, a conjugate comprising afluorophore may be covalently linked to one or more molecules of thesubstrate. In one embodiment of the invention, the luminescent assay ismore sensitive than a corresponding assay which employs the fluorophorerhodamine-110, which can be modified via an amide bond to link twoprotease substrates to the fluorophore.

A “functional equivalent” of a reference substrate is a substrate havingone or more amino acid substitutions relative to the sequence of thereference substrate, which functionally equivalent substrate isrecognized and cleaved by the same protease at a substantially similarefficiency as the reference substrate. FIG. 13 shows exemplaryfunctionally equivalent substrates for various caspases.

The increased assay sensitivity with methods employing the luminescentsubstrates of the invention is at least 2 times, more preferably 3, 4,5, 6, 7, 8, 9, or 10, or even greater, for instance, at least 15, 20,25, 30, 40, 50, 100, 200, 500, or 1000 times or more, greater than thatof an assay employing a conjugate comprising a fluorophore covalentlylinked to at least one substrate molecule or a functional equivalentthereof. Thus, the methods of the invention may detect less than 5 μU,or less, e.g., less than 1 μU, 0.5 μU or 0.2 μU of caspase in a sample.As used herein, the limit of detection means 3 standard deviations abovebackground noise (“noise” is 1 standard deviation of background andbackground is a control without caspase).

As described hereinbelow, using a substrate for caspase 3 and 7 that waslinked to either aminoluciferin or rhodamine-110, it was found that thelimit of detection for the aminoluciferin-based substrate was 0.2 to 0.5μU of purified caspase while that for the rhodamine-110-based substratewas 10 μU. As also described herein, it was found that the limit ofdetection of caspase expressing cells with the aminoluciferin-basedsubstrate was 15 cells at 1 hour while the limit of detection for therhodamine-110-based substrate was 150 cells at 1 hour. Thus, the methodsof the invention may be employed with a sample comprising purified orpartially-purified preparations of enzyme, as well as a samplecomprising a cell lysate or intact cells. Moreover, due to the increasedsensitivity of the assay of the invention, accurate background levels ofactivity, e.g., in resting cells such as those in the absence of induceror toxin, can be readily and accurately established.

The invention also provides a luminescent assay method to detect aprotease that specifically cleaves a substrate comprising aspartate. Themethod comprises contacting a sample suspected of having one or moreaspartate-specific proteases with a mixture comprising luciferase and anamino-modified aminoluciferin or a carboxy-terminal protected derivativethereof, wherein the amino group of aminoluciferin or the derivativethereof is modified so as to covalently link the substrate via a peptidebond to aminoluciferin or a carboxy-terminal protected derivativethereof. If the sample comprises a protease having aspartate as arecognition site, the substrate is cleaved at the peptide bond thatlinks the substrate comprising aspartate to aminoluciferin, yieldingaminoluciferin, a substrate for the luciferase in the mixture. Thenluminescence is detected in the sample. Preferably, the luminescentassay is more sensitive than a corresponding assay with a conjugatecomprising a fluorophore covalently linked to one or more molecules ofthe substrate or a functional equivalent thereof. Preferred proteasesthat specifically cleave a substrate comprising aspartate include butare not limited to caspases, e.g., any one of caspases 1–14. Preferredsubstrates comprise X₁-X₂-X₃-D (SEQ ID NO:19), wherein X₁ is Y, D, L, V,I, A, W, or P; X₂ is V or E; and X₃ is any amino acid, for instance, asubstrate comprising DEVD (SEQ ID NO:1), WEHD (SEQ ID NO:9), VDVAD (SEQID NO:4), LEHD (SEQ ID NO:3), VEID (SEQ ID NO:5), VEVD (SEQ ID NO:15),VEHD (SEQ ID NO:11), IETD (SEQ ID NO:6), AEVD (SEQ ID NO:7), LEXD (SEQID NO:14), VEXD (SEQ ID NO:16), IEHD (SEQ ID NO:17), or PEHD (SEQ IDNO:18).

The invention also provides a luminescent assay method to detect trypsinor tryptase. The method comprises contacting a sample suspected ofhaving trypsin or tryptase with a mixture comprising luciferase and anamino-modified aminoluciferase or a carboxy-terminal protectedderivative thereof, wherein the amino group of aminoluciferin or thederivative thereof is modified so as to covalently link a substrate fortrypsin or trytase via a peptide bond to aminoluciferin or acarboxy-terminal protected derivative thereof. Luminescence is thendetected. Preferably, the luminescent assay is more sensitive than acorresponding assay with a conjugate comprising a fluorophore covalentlylinked to at least one substrate molecule or a functional equivalentthereof. For trypsin, arginine and lysine are functionally equivalentsubstrates as trypsin cleaves the peptide bond after those residues withsubstantially similar efficiencies. The increased assay sensitivity withmethods employing the luminescent substrates of the invention fortrypsin or tryptase is at least 2 times, more preferably 3, 4, 5, 6, 7,8, 9, or 10, or even greater, for instance, at least 15, 20, 25, 30, 40,50 or 100 times or more, greater than that of an assay employing aconjugate comprising a fluorophore covalently linked to at least onesubstrate molecule or a functional equivalent thereof. Using a substratefor trypsin, it was found that the limit of detection for alysyl-aminoluciferin substrate was 3.0 pg while that for thearginine₂-rhodamine-110-based substrate was 12 to 30 pg. Thus, a trypsinassay which employs an amino-modified aminoluciferin substrate is atleast 4 times more sensitive than a corresponding assay with a conjugatecomprising rhodamine-110 covalently linked to two functionallyequivalent trypsin substrates.

Further provided is a luminescent assay method to detect a protease thatspecifically cleaves a substrate comprising arginine or lysine. Themethod comprises contacting a sample suspected of having one or moreproteases specific for a substrate comprising arginine or lysine with amixture comprising luciferase and an amino-modified aminoluciferase or acarboxy-terminal protected derivative thereof covalently linked via apeptide bond to a substrate comprising arginine or lysine. Luminescencein the sample is then detected. Preferably, the assay is more sensitivethan a corresponding assay with a conjugate comprising a fluorophorecovalently linked to the substrate or a functional equivalent of thesubstrate. As tryptase is released from activated mast cells inassociation with inflammatory conditions including allergic reactionssuch as anaphylactic reactions and allergic rhinitis, and trypsin instool may be indicative of cystic fibrosis, the methods of the inventionmay be of diagnostic use, or to monitor a mammal subjected to therapy,e.g., anti-inflammatory therapy.

Also provided is a compound comprising aminoluciferin or a carboxy-terminal protected derivative thereof covalently linked via a peptidebond to a protease recognition site such as a caspase recognition site,a trypsin recognition site, or a tryptase recognition site.

The invention also provides synthetic processes and intermediatesdisclosed herein, which are useful for preparing compounds of theinvention.

Kits useful in the methods of the invention are also envisioned. Suchkits may comprise the amino-modified aminoluciferins or carboxy-terminalprotected derivatives of the invention, and instructions for their use,optionally a luciferase, for instance a thermostable luciferase and alsooptionally a buffer for a luminescence reaction which may include alysing agent.

BRIEF DESCRIPTION OF THE FIGURES

FIG. 1 depicts relative light units (RLU) for luciferin oraminoluciferin as a substrate for a thermostable firefly luciferase in aluminescent reaction. Aminoluciferin produces about 60% of the lightoutput as luciferin under saturating conditions. The K_(m) shifts fromabout 0.6 μM for luciferin to 2 μM for aminoluciferin.

FIG. 2 illustrates the elimination of background signal from freeaminoluciferin in a homogeneous assay format. Free aminoluciferin canproduce high background signal even in the absence of trypsin. Thisbackground signal decreases as the free aminoluciferin is consumed byluciferase. By combining the substrate with luciferase, ATP, and Mg+prior to the exposure to protease, the signal to noise ratio isdramatically increased. Moreover, the presence of the protease did notinterfere with the luciferase reaction.

FIG. 3A shows RLU from a trypsin titration with N-Lys-aminoluciferinover time. Substrate was combined with luciferase, ATP, and Mg+ inbuffer and incubated overnight to eliminate free aminoluciferin. Thesubstrate mixture was then added to the trypsin titrations.

FIG. 3B shows RLU (log) from a trypsin titration withN-Lys-aminoluciferin over an extended period of time.

FIG. 4 depicts relative fluorescent units (RFU) from a trypsin titrationwith Z-Arg-Rho110.

FIG. 5 shows RLU from a caspase titration with Z-DEVD-aminoluciferinover time. Luciferase, ATP, and Mg+ in buffer were added to thesubstrate.

FIG. 6 shows RFU from a caspase titration with Z-DEVD-Rho110. Thefluorescent Z-DEVD-Rho110 substrate was provided in the Apo-ONE™Homogeneous Caspase 3/7 Assay kit (Promega). The same buffer was usedfor the DEVD-Rho110 substrate as for the DEVD-aminoluciferin substrate(see FIG. 5).

FIG. 7 shows RFU (background subtracted) or RFU (log) for caspase andZ-DEVD-Rho110.

FIG. 8 shows RLU (background subtracted) or RLU (log) for caspase andZ-DEVD-aminoluciferin.

FIG. 9 is a comparison of RLU and RFU for trypsin withN-Lys-aminoluciferin or Z-Arg-Rho110 as a substrate. Trypsin titrationswere set up as described above. The luminescent assay was more sensitivethan a comparable fluorescent assay, e.g., the Lys-aminoluciferinsubstrate has a sensitivity 3-10 fold greater than the Arg-Rho110depending on the time of the reading.

FIG. 10 is a comparison of RLU and RFU for caspase andZ-DEVD-aminoluciferin, Z-DEVD-AMC or Z-DEVD-Rho110 as a substrate. TheZ-DEVD-Rho110 substrate and buffer used were the Apo-ONE™ (Promega)substrate and buffer. The same buffer was used for the DEVD-AMCsubstrate. The DEVD-aminoluciferin substrate had a sensitivity 50-300fold greater than the Arg-Rho110 depending on the time of the readings.

FIG. 11 shows RLU or RFU obtained with Jurkat cells (induced oruninduced) and the caspase substrate Z-DEVD-aminoluciferin orZ-DEVD-Rho110. The Apo-ONE™ buffer and Z-DEVD-Rho110 substrate were usedfor the fluorescent caspase assay. The same buffer was used for theDEVD-aminoluciferin substrate with the addition of luciferase, ATP, andMgSO₄. The DEVD-aminoluciferin substrate has a sensitivity 10-foldgreater than the Arg-Rho110 at the 1 hour time point. At 4 hours, thisdecreased to about 2-fold.

FIG. 12 shows relative RLU or RFU results obtained using CHAPS orApo-ONE™ buffer.

FIG. 13 illustrates recognition sites for various caspases (SEQ IDNOs:1–18; Thornberry et al., 1997; Garcia-Calvo et al., 1999).

DETAILED DESCRIPTION OF THE INVENTION

Rapid and sensitive assays of proteolytic activity are important forgeneral characterization of proteases and high-throughput screening forprotease inhibitors. However, the inherent background of fluorescence,particularly in cell-based systems, can limit assay sensitivity.Moreover, to achieve maximum sensitivity, lengthy incubations are oftenrequired for accumulating the fluorescent assay product. Luminescentassays can often provide greater sensitivity in less time.

Thus, the present invention provides an improved, sensitive method formonitoring protease activity in purified preparations comprising theprotease, in cell lysates or cells, either prokaryotic or eukaryoticcells. Preferred eukaryotic cells include mammalian cells, for example,human, feline, bovine, canine, caprine, ovine, swine, equine, non-humanprimate, e.g., simian, avian, plant or insect cells. The cells may becells that have not been genetically modified via recombinant techniques(nonrecombinant cells), or recombinant cells, the genome of which isaugmented with a recombinant DNA. The DNA may encode a protease to bedetected by the methods of the invention, a molecule which alters thelevel or activity of the protease in the cell, and/or a moleculeunrelated to the protease or molecules that alter the level or activityof the protease.

The protease is detected using an amino-modified aminoluciferin or acarboxy-terminal protected derivative thereof, which modificationcomprises a substrate for the protease. The substrate, which comprisesone or more amino acid residues which include the recognition site forthe protease, is covalently linked to the amino group of aminoluciferinor the carboxy-terminal modified derivative via a peptide bond.Preferably, the N-terminus of the substrate is modified to preventdegradation by aminopeptidases, e.g., using an amino-terminal protectinggroup.

In the absence of the appropriate enzyme, a mixture comprising asubstrate and luciferase will generate minimal light as minimalaminoluciferin is present (a small amount of light may be generated dueto spontaneous hydrolysis of the peptide bond). In the presence of theappropriate enzyme, the peptide bond linking the substrate andaminoluciferin (the bond immediately adjacent to the 6′ position on theluciferin core molecule) can be cleaved by the protease to yieldaminoluciferin, a substrate for luciferase. Thus, in the presence ofluciferase, for instance, a native, a recombinant or a mutantluciferase, light is generated, which is proportional to the amount oractivity of the protease. Any beetle luciferase, preferably athermostable luciferase, may be employed in the methods of theinvention.

The aminoluciferin-based substrates of the invention are relativelyinexpensive to synthesize and can be purified to high levels. Moreover,because they are extremely sensitive substrates, only very small amountsof a biological sample (e.g., cells, and physiological fluids, blood,urine, etc., which comprise cells) are required to perform the assay.Further, because the aminoluciferin-based substrates are extremelyselective, little or no purification of the biological sample isrequired. For example, using such an assay, the activity of caspase 3,caspase 7 and trypsin was found to be below the level of detection of acorresponding assay using a Rhodamine-110 caspase substrate(Rhodamine-110 is likely one of the most sensitive indicators known). Inparticular, the sensitivity described herein for a caspase is superiorto Apo-ONE™ (Promega, Madison, Wis.). Apo-ONE™ is a fluorescent basedassay, which uses the fluorphore Rhodamine-110 conjugated to 2recognition sequences for caspase 3/7.

Preferably, the methods of the invention are employed as a homogeneousassay for a protease, such as a caspase, tryptase or trypsin, i.e., themodified aminoluciferin, luciferase and additional components are mixedprior to adding the mixture to the sample. Results may be read withoutadditional transfer of reagents.

A specific compound of the invention is a compound of formula (I):

wherein R is a peptide that is a substrate for caspase, trypsin andtryptase, which is linked to the remainder of the compound of formula(I) through its C-terminus forming a peptide (amide) bond; and R′ is Hor a suitable carboxy protecting group (e.g. a (C₁–C₆)alkyl, phenyl orbenzyl ester), or a suitable salt thereof.

Another specific compound of the invention is a compound of formula (I):

wherein R is a peptide that is linked to the remainder of the compoundof formula (I) through an aspartate, lysine, or arginine group at theC-terminus of the peptide forming a peptide (amide) bond; and R′ is H ora suitable carboxy protecting group (e.g. a (C₁–_(C) ₆)alkyl, phenyl orbenzyl ester), or a suitable salt thereof.

It will be appreciated that salts of the amino-modified aminoluciferincompounds or the carboxy-terminal protected derivatives thereof can alsobe used in the methods described herein, and also form part of theinvention. Methods for preparing suitable salts are known in the art.

Compounds of the invention can be prepared using procedures that aregenerally known, or they can be prepared using the procedures describedherein. For example, compounds of the invention can be prepared usingstandard solution phase chemistry. Accordingly, a peptide can be coupledto an amino-cyanobenzothiazole, followed by reaction with D-cysteine toprovide a compound of the invention. Alternatively,amino-cyanobenzothiazole can first be reacted with D-cysteine to providean intermediate amino compound, which can subsequently be conjugated toa peptide to provide a compound of the invention.

Compounds of the invention can also be prepared using conventionalsolid-phase peptide synthesis techniques. For example, an aminoluciferinlabeling reagent in the form of an N-protected amino acid that isattached to a peptide synthesis resin via the carboxylic acid function,can be prepared using standard coupling reagents (e.g., EDAC, DCC, orHOBt). The N-protective group is preferably Fmoc or t-Boc, but can beany group that can be removed without deleterious effect on the chemicalbond connecting the label to the resin. Once attached to the resin theN-protective group is removed and the peptide is built onto theN-terminus of the resin-bound label using standard peptide synthesisprotocols. At the end of peptide synthesis the labeled peptide iscleaved from the resin using standard cleavage reagents to provide thecarboxylic acid.

Accordingly, the invention provides aminoluciferin coupled via the freecarboxyl group to a solid support for the purposes of peptide synthesis.Such a carboxy-terminal protected aminoluciferin is convenient for thesynthesis of a conjugate comprising a peptide of interest conjugated tothe amino group of aminoluciferin. Preferably, the amino group isprotected with an Fmoc or a t-Boc group.

The invention also provides a method for preparing a compound of theinvention comprising forming an amide bond between the amino group of asolid support bound aminoluciferin and a first amino acid or a firstpeptide; and optionally attaching one or more additional amino acids orpeptides through peptide bonds to provide the compound. The solidsupport bound aminoluciferin can optionally be prepared by attaching anN-protected aminoluciferin to a solid support through the carboxy group;and deprotecting the aminoluciferin. The support bound compound can thenbe removed to provide the corresponding free carboxylic acid, which canoptionally be protected to provide a carboxyterminal protectedderivative.

A carboxy-protected derivative of the invention can be prepared from thecorresponding carboxylic acid using standard techniques. Accordingly,the invention provides a method to prepare a carboxy-terminal protectedderivative of aminoluciferin, comprising protecting the correspondingacid with a suitable carboxy-protecting group.

Suitable amino protecting groups (e.g. Fmoc or t-Boc), as well assuitable carboxy protecting groups (e.g. (C₁–C₆)alkyl, phenyl or benzylesters or amides) that can be incorporated into the compounds of theinvention, are well known to those skilled in the art (See for example,T. W. Greene, Protecting Groups In Organic Synthesis; Wiley: New York,1981, and references cited therein).

The invention will be further described by the following non-limitingexamples.

EXAMPLE 1

To compare the limit of detection for a luminescence-based and afluorescence-based assay for trypsin, two substrates, Lys-aminoluciferin(Cbz-modified lysinyl-aminoluciferin) and Arg-Rho-110 (Molecular Probes,Catalog no. R6501), were used. Substrate was resuspended in 100 mMHepes, pH 7.9, at a concentration of 10 mM and stored at −20° C. Thethawed Lys-aminoluciferin substrate, thermostable luciferase (5.2 mg/mlstock), and ATP (0.1 M stock) were diluted in buffer (50 mM HEPES, pH7.9, 10 mM MgSO₄, 1 mM EDTA, pH 8.2 and 0.1% prionex) to make a stockthat was 10× the final concentration. The 10× stock was 200 μMLys-aminoluciferin, 200 μg/ml luciferase, and 2.0 mM ATP. This 10× stockwas incubated for at least 90 minutes to eliminate any freeaminoluciferin.

Trypsin was prepared for titration as follows: 1 μg/μl stock solutionwas diluted to 10 ng/50 μl in the same buffer as above (50 mM HEPES, pH7.9, 10 mM MgSO₄, 1 mM EDTA, pH 8.2 and 0.1% prionex). This 10 ng/μltrypsin solution was serially diluted 5 fold to 2 ng, 0.4 ng, 0.08 ng,0.016 ng, 3.2 pg, 0.64 pg, 0.128 pg, 0.0256 pg and 0.005 pg. The trypsindilutions were added to two 96-well plates in replicates of 8 at 50 μlper well. Pipette tips were changed for each row to avoid enzymecarryover. Two columns (16 wells) contained buffer only without trypsin.

One of the two plates of the trypsin dilutions was then used to test theLys-aminoluciferin substrate (FIG. 3). Samples were tested inquadruplicate. The 10× substrate/luciferase/ATP mix was further diluted5 fold in the above-described buffer to make a 2× stock. 50 μl of this2× stock was added to each well containing 50 μl of the trypsintitration such that the final 100 μl volume contained 20 μMLys-aminoluciferin, 200 μM ATP, and 20 μg/ml of thermostable luciferasein buffer (50 mM HEPES, pH 7.9, 10 mM MgSO₄, 1 mM EDTA, pH 8.2 and 0.1%prionex). The substrate mix was also added to 12 of the 16 wellscontaining buffer only without trypsin. The remaining 4 wells were leftwith buffer only (no substrate mix). This plate was incubated at roomtemperature and read by luminometer at 45 minutes and 3 hours afteradding the substrate mix to the trypsin.

The second plate of trypsin dilutions was used to test the Arg-Rho-110substrate (FIG. 4). The Arg-Rho-110 substrate was tested at finalconcentrations of 10 μM and 2.5 μM. 2× stocks of 20 μM and 5 μM wereprepared by diluting the substrate in 50 mM HEPES, pH 7.9, 10 mM MgSO₄,1 mM EDTA, pH 8.2 and 0.1% prionex. To each well of the plate containing50 μl of the trypsin titration was added 50 μl of either the 20 μM or 5μM 2× stocks of Arg-Rho-110 substrate. The 20 μM stock was added to thefirst four rows (final concentration of 10 μM in rows A-D) and the 5 μMstock was added to the second four rows (final concentration of 2.5 μMin rows E-H). The substrate mix was also added to 12 of the 16 wellscontaining buffer only without trypsin. The remaining 4 wells were leftwith buffer only (no substrate mix). The Arg-Rho-110 plate was incubatedat room temperature in the dark for 4.5 hours and read on a fluorimeter.

The signal to noise was calculated as signal-background (notrypsin)/S.D. of background. The limit of detection was determined as 3S.D. above background noise.

Results

A homogeneous format was used for a trypsin assay. The results indicatedthat the limit of detection using the Lys-aminoluciferin substrate fortrypsin is lower than the Arg-Rho-110 substrate. In particular, theLys-aminoluciferin substrate has a sensitivity 3 to 10 fold greaterdepending on the time of the reading than the Arg-Rho-110 substrate(FIG. 9). Moreover, the luminescent assay reached a maximum sensitivityin 30 minutes or less and was very stable for extended time periods.

EXAMPLE 2

To determine the effect of an overnight pre-incubation of substrate withluciferase, ATP and buffer prior to adding trypsin, asubstrate/luciferase/ATP mix was incubated overnight in the dark, atroom temperature. For the Lys-aminoluciferin substrate, the thawedLys-aminoluciferin substrate (10 mM stock), thermostable luciferase (5.2mg/ml stock), and ATP (0.1 M stock) were diluted in buffer (50 mM HEPES,pH 7.9, 10 mM MgSO₄, 1 mM EDTA, pH 8.2 and 0.1% prionex) to make a stockthat was 10× the final concentration. The 10× stock was 200 μMLys-aminoluciferin, 200 μg/ml luciferase, and 2.0 mM ATP. Afterovernight incubation, the 10× stock was diluted in buffer to make a 2×stock (40 μM of substrate, 400 μM of ATP and 40 μg/ml of luciferase).The Arg-Rho-110 substrate was also prepared to a 2× working stockconcentration of 40 μM from a 5 mM stock.

Trypsin dilutions were prepared from a 1 μg/μl stock and diluted to thesame concentrations as in Example 1, and two different plates were setup with 4 wells for each concentration of trypsin, 50 μl per well. Twocolumns had buffer only without trypsin as a control. Then, 50 μl of the2× Lys-aminoluciferin substrate mix was added to each well of one plate,and the results were read at several time points on a luminometer. Tothe second plate, 50 μl of the 2× Arg-Rho-110 stock was added to eachwell for a final concentration of 20 μM, as in Example 1.

The luminescence-based assay was able to detect as little as 3.0 pg oftrypsin, while the fluorescence-based assay had a limit of detection ofabout 12–30 pg (4–10 times less enzyme).

EXAMPLE 3

To conduct a direct comparison between luminescent and fluorescentsubstrates for caspase 3, DEVD-Rho-110 and DEVD-aminoluciferin wereemployed. The DEVD-aminoluciferin substrate/luciferase/ATP mixture wasprepared first and preincubated prior to the enzyme assay to eliminatefree aminoluciferin. To 1.25 ml of Apo-ONE™ buffer (Promega) was added10 μl of DEVD-luciferin (10 mM stock), 10 pL of ATP (0.1 M stock), 50 μlof MgSO₄ (1 M stock), 50 μl of prionex (10% stock), and 48 μl ofluciferase (5.2 mg/ml stock). The volume was brought up to 2.5 ml withnanopure, autoclaved water to make a 2× stock of 40 μM DEVD-luciferin,400 pM ATP, 0.2% prionex, and 100 μg/ml luciferase. This stock wasincubated overnight at room temperature.

Caspase (Upstate Biotech, Cat. No.14–264; approximately 10 mU/ng proteinwith >75% in active conformation) was diluted 550 fold in a 50/50mixture of Apo-ONE™ buffer/RPMI-1640 culture media, from 1 U/pl to 1.8mU/μl. 50 μl of Apo-One™ buffer/RPMI-1640 media was added to each wellin two 96-well plates. Then, 5.5 μl of the caspase stock was added tothe 50 il of Apo-One™ buffer/RPMI-1640 media for a concentration ofabout 10 mU/50 μl. Ten serial dilutions of 10 fold each were carried outin the wells by serially transferring 5.5 μl of caspase solution intothe 50 μl of the buffer/media mix. The final caspase concentrations were10 mU, 1 mU, 0.1 mU, 0.01 mU, 0.001 mU, 0.1 μU, 0.01 μU, 0.001 μU, 0.1nU, and 0.01 nU/well. The last two columns were buffer/media onlywithout caspase.

To one plate of caspase dilutions, 50 μl of the DEVD-aminoluciferinsubstrate mixture was added to each well for final concentrations of 20μM DEVD-aminoluciferin, 200 μM ATP, 10 mM MgSO₄, 0.1% prionex, and 50μg/ml luciferase. To the other plate, 50 μl of the DEVD-Rho-110substrate was added to each well at the recommended final concentrationof 50 μM. Readings were taken for each plate at 1 hour, 3 hours, and 5hours on a luminometer and fluorimeter, respectively.

Signal to noise was calculated as above and the limit of detection wasdetermined as 3 S.D. above the background noise.

As in the case of the trypsin-substrate modified luciferin,DEVD-aminoluciferin was 10–100 times more sensitive than DEVD-Rho-110(FIG. 10). The fluorescent ratio assay required several hours formaximum sensitivity and was always changing over time. Moreover, thefluorescent assay lost linearity at low caspase concentrations.

The luminescent assay is a rate assay that is not dependent on theaccumulation of cleaved substrate. Therefore, steady-state (proteasecleavage versus luciferase consumption of aminoluciferin) is reachedrapidly and this steady-state is stable for several hours. Moreover,linearity is also maintained for several hours. The luminescent assayreached a maximum sensitivity in 30 minutes or less and was very stablefor extended time periods. The luminescent assay was linear over 3–4logs at low caspase concentration (FIG. 8).

EXAMPLE 4

The DEVD-aminoluciferin caspase substrate and the DEVD-Rho-110 caspasesubstrate were used to measure caspase activity in Jurkat cells inducedto undergo apoptosis with anti-FAS antibody. DEVD-luciferin andluciferase were prepared for pre-incubation prior to use in the assay.Substrate, ATP, MgSO₄, prionex and luciferase were diluted from the samestock as in Example 3 to the same 2× concentration, except that thecomponents were diluted in autoclaved water rather than Apo-ONE™. Thismixture was incubated overnight in the dark (covered in foil).

The next day, Jurkat cells, grown in RPMI-1640 media with 10% FetalBovine Serum (FBS) to a density of 5×10⁵ cells/ml, were treated withanti-FAS antibody. To one vial of 8 ml of media was added 1.6 μl ofantibody (1:5000 dilution); a second vial contained 8 ml of media and noantibody. Cells were incubated for 4 hours at 37° C., in 5% CO₂. Cellswere then centrifuged and resuspended in 12.5 ml of RPMI-1640 to adensity of 3.2×10⁵ cells/ml, then diluted 1:1 with Apo-ONE™ buffer for1.6×10⁵ cells/ml, or 8,000 per 50 μl.

Two 96-well plates were prepared such that on each, the first column wasleft empty, and 50 μl of RPMI-1640:Apo-ONE™ solution was placed in eachof the remaining wells. To the first four rows of each plate, 100 μl ofthe “induced” cell solution (8,000 cells) was added and the cells thenserially diluted from 8,000 cells/well, to 4,000 cells/well and so on,down to 7.8 cells/well. The twelfth column was left without cells andwith only 50 μl of RPMI-1640:Apo-ONE™ solution. The next four rows oneach plate were likewise treated with 100 μl of the “uninduced” cellsolution, and then likewise serially diluted to 7.8 cells/well. Again,the last column of those four rows was left with media alone (no cells).

Signal to noise was calculated (signal minus background (no cellcontrol)/S.D. of background) and the limit of detection was determinedas 3 S.D. above the background noise.

One of the two plates was treated with 50 μl of eitherDEVD-aminoluciferin/luciferase mix or with DEVD-Rho-110/Apo-ONE™(Promega G778B and G777B). Each plate was mixed on a plate shaker for 30seconds then incubated at room temperature and read on either a Dynexluminometer or Fluoroskan plate reader at 1 hour, 2 hours, 4 hours andone day later.

The aminoluciferin substrate assay showed that the assay detects caspasepositive cells in a well having 15 cells in as little as 1 hour, andthat the assay remains linear at the 4 hour time point (FIG. 11). On theother hand, the DEVD-Rho-110 substrate assay had a limit of detection of150 cells/well at 1 hour, and about 30 cells at 4 hours.

EXAMPLE 5

To evaluate different assay component formulations for caspase-3activity and to compare sensitivities of DEVD-aminoluciferin toDEVD-Rho-110 in those formulations, two stock solutions were prepared.As above, the DEVD-aminoluciferin/luciferase mixture is prepared andallowed to incubate overnight. One stock solution included a 1% solutionof CHAPS buffer (Sigma, Catalog No. C-5070) and the other a 1% solutionof Thesit (Pragmatics, Inc., Catalog No. S-22#9). The bufferformulations were as follows: 100 μl of HEPES (1 M stock, 50 mM finalconcentration), 10 μl of CaCl₂ (1 M stock, 5 mM final concentration), 30μl of MgSO₄ (1 M stock, 15 mM final concentration), 8 μl of ATP (0.1 Mstock, 400 μM final concentration), 8 μl of DEVD-aminoluciferin (10 mMstock, 40 μM final concentration), 38.4 μl of luciferase (5.2 mg/mlstock, 100 μg/ml final concentration) and 20 μl of prionex (10% stock,0.1% final concentration) were combined in each of 2 tubes. Finally, 200μl of either CHAPS or Thesit (1% stock, 0.1% final concentration) wasadded to one of the tubes. This was incubated overnight. The next day,40 μl of DTT (Promega, catalog no. V3151) (1 M stock, 20 mM finalconcentration) was added to each tube (CHAPS and Thesit). Finally,1545.6 μl of pure, autoclaved water was added for a final volume of 2 mlper solution.

Caspase (Upstate Biotech, Cat. No. 14-264) was diluted from 1 U/μl stockto 1 mU per 50 μl, or 8 mU in 400 μl of buffer (see below). The caspasebuffer was as follows: HEPES, CHAPS or Thesit, CaCl₂, MgSO₄, DTT andprionex, all in the same final concentrations as described above. Thecaspase was serially diluted by factors of 10, from 1 mU through 1×10⁻⁸mU to a final volume of 440 μL of each dilution. 50 μl of each of thesedilute caspase solutions were added to each of 3 wells on each of two 96well plates. Three columns of wells were left blank.

To one plate, 50 μl of DEVD-aminoluciferin/luciferase was added to eachof the 6 wells containing each dilution (both CHAPS and Thesit treated).To the second plate, 50 μl of DEVD-Rho-110 substrate was added to eachof the 6 wells containing each dilution (both CHAPS and Thesit treated).The plates were read at various times on a luminometer (Dynex) or afluorimeter (Fluoroskan).

The limit of detection for the aminoluciferin was in the range of 0.2 μUof caspase in buffer containing either CHAPS or Thesit (FIG. 12). On theother hand, the limit of detection using the rhodamine-based substratewas 2–6 μU (a factor of 10–30).

EXAMPLE 6

Representative compounds of the invention were prepared according to thefollowing non-limiting examples.

General Synthetic Procedures

All reactions were run under positive pressure of dry nitrogen gas.Reactions requiring anhydrous conditions were performed in oven-driedglassware that was cooled under nitrogen gas or a dessicator. Anhydroussolvents and reagent solutions were transferred using oven-driedsyringes. Tetrahydrofuran (THF), dichloromethane, pyridine,acetonitrile, and dimethylformamide (DMF) were obtained as anhydroussolvent and were used without further purification. Reagent gradesolvents were used for chromatography without further purification.

TLC was performed on 0.2mm EM Science precoated silica gel 60 F₂₅₄ TLCplates (5×20 cm aluminum sheets). Flash chromatography was performedusing Selecto Scientific 32–63 μm silica gel (60 F₂₅₄).

Analytical Reverse-phase HPLC was performed using a Synergi 4 μ Max-RPcolumn, 4.6 mm×50 mm, on Beckman System Gold 126 pump systems equippedwith a Model 168 diode-array detector and Model 507 autosampler. Thesolvents were: A—10 mM sodium phosphate buffer (pH 7.0) and B—methanol.All analytical reverse-phase chromatograms were monitored at 254 nm and315 nm.

ESI Mass spectra were recorded on a FISONS VG Platform Electrospray MassSpectrometer. The NMR spectra of all the compounds conformed to theirrespective structures.

Nmr spectra were obtained on a Varian 300 Mhz spectrometer.

EXAMPLE 7 Preparation of N-(Z-DEVD)-Aminoluciferin

To a 25 mL flask containing N-[Z-Asp(OtBu)-Glu(OtBu)-Val-Asp(OtBu)]-aminoluciferin (20 mg, 0.019 mmol) was added 2 mL of a solution of 20%trifluoroacetic acid in dichloromethane. The reaction mixture wasstirred at room temperature for 4 h. HPLC indicated the reaction wasprogressing slowly. Additional trifluoroacetic acid was added to make a30% solution of trifluoroacetic acid in the reaction mixture and thereaction was left standing in a 5° C. refrigerator overnight. The nextday HPLC analysis indicated the reaction was complete. The reactionmixture was concentrated to a creamy solid residue. The crude productwas purified by HPLC chromatography on a Synergi 4 u Max-RPsemi-preparative column using 20mM ammonium acetate buffer (pH 6.5) andmethanol. Fractions containing product were pooled and lyophilized toafford 10.3 mg (62%) of N-(Z-Asp-Glu-Val-Asp)-aminoluciferin as anoff-white powder.

The intermediate N-[Z-Asp(OtBu)-Glu(OtBu)-Val-Asp(OtBu)]-aminoluciferinwas prepared as follows.

a. Synthesis of Asp(OtBuj-OH. Fmoc-Asp(OtBu)-OH (500 mg, 1.2 mmole) wasdissolved in a 9:1 mixture of dichloromethane-piperidine (5 mL) in a 25mL round-bottomed flask. The reaction mixture was stirred overnight atroom temperature. The next morning, TLC analysis indicated complete Fmocdeprotection. The reaction mixture was concentrated by rotoevaporation,coevaporated 2 times with toluene, and dried under vacuum to give acrude oil. This oil was purified by flash chromatography on silica gel(50 g) using a stepwise solvent gradient of 10%–50% methanol indichloromethane to afford 250 mg (100%) of Asp(OtBu)-OH.

b. Synthesis of Fmoc-Val-Asp(OtBu)-OH. Asp(OtBu)-OH (250 mg, 1.3 mmol)was dissolved in 40 mL of a 1:1 mixture of dichloromethane and pyridinein a 100 mL round-bottomed flask with magnetic stirring. To thissolution was added Fmoc-Val-OSu (690 mg, 1.58 mmole) and stirringcontinued overnight under nitrogen atmosphere at ambient temperature.The next morning the reaction was concentrated by rotoevaporation to apale yellow oil, which was dissolved in dichloromethane and washed twicewith 10% aqueous citric acid solution. The aqueous layer was extractedagain with dichloromethane, and the combined organic layer was driedwith anhydrous sodium sulfate and concentrated by rotoevaporation togive 820 mg of crude white foam. This material was purified by flashchromatography on silica gel using a step-wise solvent gradient of3%–20% methanol in dichloromethane to afford 250 mg (38%) ofFmoc-Val-Asp(OtBu)-OH as an off-white solid.

c. Synthesis of Val-Asp(OtBu)-OH. Fmoc-Val-Asp(OtBu)-OH (250 mg, 0.57mmole) was dissolved in 10 mL of a 9:1 mixture ofpiperidine-dichloromethane in a 50 mL round-bottomed flask, and thereaction mixture was allowed to stand at ambient temperature. After 1 hTLC analysis indicated the reaction was complete. The reaction mixturewas concentrated, coevaporated twice with 20 mL of toluene, and driedunder vacuum to provide 250 mg of a crude white residue. This residuewas purified by flash chromatography on silica gel (25 g) using astep-wise solvent gradient of 20%–75% methanol in dichloromethane toafford 120 mg (76%) of pure Val-Asp(OtBu)-OH.

d. Synthesis of Z-Asp(OtBu)-Glu(OtBu)-OH. To a stirred suspension ofGlu(OtBu)-OH in dichloromethane (5 mL) and pyridine (3 mL) in a 25 mLround-bottomed flask was added Z-Asp(OtBu)-Osu (530 mg, 1.26 mmol). Theresulting mixture was stirred at room temperature for two days. Thereaction mixture was concentrated by rotoevaporation and the residue waspartitioned between ethyl acetate and 10% aqueous citric acid solution.The aqueous phase was extracted three times with ethyl acetate. Combinedextracts were dried over sodium sulfate and concentrated byrotoevaporation to give a crude oil that was purified by flashchromatography on silica gel (75 g) using a stepwise solvent gradient of2%–4% methanol in dichloromethane. Fractions containing product werepooled and concentrated by rotoevaporation to give 630 mg (98%) ofZ-Asp(OtBu)-Glu(OtBu)-OH as an off-white solid foam.

e. Synthesis of Z-Asp(OtBu)-Glu(OtBu)-Val-Asp(OtBu)-OH. To a stirredsolution of Z-Asp(OtBu)-Glu(OtBu)-OH (245 mg, 0.48 mmol) indichloromethane (20 mL) in a 100 mL round-bottomed flask was addedN-hydroxysuccinimide (60.9 mg, 0.53 mmole) followed bydicyclohexylcarbodiimide (109.2 mg., 0.53 mmol), and the resultingcloudy mixture was allowed to stir for 1 h at ambient temperature undernitrogen atmosphere. After TLC analysis showed the reaction wascomplete, the dicyclohexylurea precipitate was removed by filtration andthe filtrate was concentrated by rotoevaporation to about 7 mL, at whichpoint some precipitation began to occur. This mixture was added to astirred solution of Asp(OtBu)-Val-OH (120 mg., 0.437 mmol) in DMF (20mL) in a 50 mL round-bottomed flask. After the reaction was stirred 2 hat ambient temperature, TLC analysis indicated the reaction wasproceeding slowly, and the hazy mixture was then concentrated to about15 mL and stirring was continued overnight. The next morning, TLCanalysis showed the reaction was not yet complete. The reaction flaskwas fitted with a condenser and the mixture was then heated at 35–38° C.using a water bath for 1.5 h, during which time the mixture clarifiedsomewhat. The reaction was cooled and concentrated by rotoevaporation toa residue, which was suspended in ethyl acetate (50 mL) and washed twicewith 10 mL of 10% aqueous citric acid solution. The organic layer wasthen washed twice with 10 mL of water, and the water layer wasback-extracted with ethyl acetate. The organic layers were combined,dried over anhydrous sodium sulfate, concentrated by rotoevaporation,and coevaporated twice with dichloromethane to afford 500 mg of crudeoff-white foam. This crude material was purified by flash chromatographyon silica gel (25 g) using a step-wise solvent gradient of 5%–20%methanol in dichloromethane to provide 190 mg (56%) ofZ-Asp(OtBu)-Glu(OtBu)-Val-Asp(OtBu)-OH as an off-white foam.

f. Synthesis of6-(Z-Asp(OtBu)-Glu(OtBu)-Val-Asp(OtBu)-amino-2-cyanobenzothiazole. To astirred solution of of Z-Asp(OtBu)-Glu(OtBu)-Val-Asp(OtBu)-OH (95 mg,0.112 mmol) in THF (5 mL) cooled in −10° C. bath (sodium chloride-ice)was added via syringe N-methylmorpholine (13.4 μL, 0.122 mmol) and thenisobutyl chloroformate (16 μL, 0.122 mmol). The reaction mixture wasstirred at −10° C. for 1 h and then a solution of6-amino-2-cyanobenzothiazole (27.9 mg, 0.159 mmol) in THF (2 mL) wasadded via pipet. The cooling bath was removed and the reaction mixturewas allowed to warm to room temperature and stir for 2 days. TLCanalysis indicated a new product was generated. The reaction mixture wasconcentrated by rotoevaporation to give a crude residue that wasdissolved in ethyl acetate and washed twice with water. The aqueousphase was extracted again with ethyl acetate. Combined extracts weredried and concentrated to give 104 mg of crude yellow residue that waspurifies by flash chromatography on silica gel (10 g) using a stepwisesolvent gradient of 4%–7% acetone in dichloromethane to provide 44 mg(42%) of6-(Z-Asp(OtBu)-Glu(OtBu)-Val-Asp(OtBu)-amino-2-cyanobenzothiazole as ayellow

g. Synthesis of N-[Z-Asp(OtBu)-Glu(OtBu)-Val-Asp(OtBu)]-aminoluciferin.To a 50 mL round-bottomed flask was added D-cysteine hydrochloride (997mg, 5.68 mmol) and deionized water (14 mL, degassed with bubblingnitrogen for 15 min). The resulting mixture was magnetically stirredunder nitrogen atmosphere until dissolution was achieved. To a separate25mL erlenmeyer flask was added anhydrous potassium carbonate (785 mg,5.68 mmol) and degassed deionized water (14 mL). The resulting solutionwas added in portions via pasteur pipet to the flask containing theD-cysteine solution, with periodic addition of 6N hydrochloric acid asneeded to maintain a pH less than 7.0. After addition of the potassiumcarbonate solution to the reaction flask was complete, a portion of thereaction mixture (0.24 mL, containing approximately 0.047 mmoles ofD-cysteine) was measured (via pipet) and transferred to a separate 5 mLreaction vial. A solution of6-(Z-Asp(OtBu)-Glu(OtBu)-Val-Asp(OtBu)-amino-2-cyanobenzothiazole (44mg, 0.047) in methanol (1 mL) was then added to the 5 mL reaction vial,followed by addition of 0.1 M hydrochloric acid solution as needed tomaintain the pH below 7.0. The reaction mixture was stirred at roomtemperature overnight. HPLC and TLC analysis indicated consumption ofstarting materials. The reaction mixture was concentrated byrotoevaporation and coevaporated with acetonitrile to give a solidresidue. The crude product was chromatographed on silica gel (10 g)using a stepwise solvent gradient of 10%–12% methanol in dichloromethaneto afford 20 mg (41%) ofN-[Z-Asp(OtBu)-Glu(OtBu)-Val-Asp(OtBu)]-aminoluciferin as an off-whitesolid.

EXAMPLE 8

Preparation of Racemic N-Fmoc-Aminoluciferin.

2-[6′-(9-fluorenylmethoxycarbonyl)amino-2′-benzothiazolyl]-Δ²-thiazoline-4-carboxylicacid (N-Fmoc-aminoluciferin). To a 100 mL round-bottomed flask wereadded N-trifluoroacetyl-aminoluciferin (660 mg, 1.76 mmol) and asolution of methanolic ammonia (30 mL of a 7 M solution, 210 mmol). Theresulting mixture was left standing at room temperature for 4 days. Thereaction mixture was concentrated by rotoevaporation and thencoevaporated with dichloromethane to give 626 mg of a crude brown solidresidue that was used in the next step without purification. A portionof the crude brown solid residue (391 mg) was dissolved in methanol (40mL) and water (2 mL) in a 100 mL round-bottomed flask. To this solutionwas added Fmoc-Cl (435 mg, 1.68 mmol) and the reaction mixture wasstirred at room temperature for 2 h. The reaction mixture wasconcentrated by rotoevaporation and coevaporated with acetonitrile andthen dichloromethane to afford a brown foam after drying under vacuum.The product was purified by flash chromatography on 3 successive silicagel columns using 100 g for the first 2 columns and 150 g for the third.The eluting solvent for the first column was 98:2dichloromethane-methanol. The eluting solvent for the second column was93:7 dichloromethane-methanol. The eluting solvent for the third columnwas 97:3 dichloromethane-methanol. Fractions containing product werecombined and concentrated to provide 280 mg of waxy product that was 95%pure and 320 mg of product that was 89% pure. The 280 mg of waxymaterial was re-purified on 25 g of silica gel using 4:1dichloromethane-methanol to give 108 mg of dry pale yellow solid. The320 mg portion of product was combined with 230 mg of material recoveredfrom combined impure fractions from the 3 columns described above. Thiscombined material (550 mg) was re-purified by flash chromatography onsilica gel (50 g) using 78:22 dichloromethane-methanol to afford 223 mgof product. Total product thus obtained was 380 mg of amber solid thatwas 95% pure by HPLC analysis. MS (ESI⁻) m/z 500 (M−H)⁻, 456 (M−H—CO₂)⁻.

The intermediate compound N-trifluoroacetyl-aminoluciferin was preparedas follows.

a. Preparation of2-(6′-Trifluoroacetylamino-2′-benzothiazolyl)-Δ²-thiazoline-4-carboxylicacid (N-Trifluoroacetyl-aminoluciferin). To a 50-mL round-bottomed flaskwas added D, L-cysteine (688 mg, 5.68 mmol) and deionized water (14 mL,degassed with bubbling nitrogen for 15 min). The resulting mixture wasmagnetically stirred until dissolution was achieved. To a separate 25-mLerlenmeyer flask was added anhydrous potassium carbonate (785 mg, 5.68mmol) and degassed deionized water (14 mL). The resulting solution wasadded in portions via pasteur pipet to the flask containing the D,L-cysteine solution, with periodic addition of 6N hydrochloric acid asneeded to maintain a pH less than 7.5. After addition of the potassiumcarbonate solution to the reaction flask was complete, a portion of thereaction mixture (10.1 mL, containing approximately 2.04 mmoles of D,L-cysteine) was measured (via graduated cylinder) and transferred to aseparate 50 mL erlenmeyer flask. This mixture was diluted with 15 mL ofdegassed methanol and the resulting solution was transferred to a 100 mLround-bottomed flask containing a solution of2-cyano-6-trifluoroacetylaminobenzothiazole (White et al., 1966) (552mg, 2.04 mmol) in degassed methanol (17 mL). The reaction mixture wasmagnetically stirred at room temperature and the reaction flask wascovered with aluminum foil. After stirring for 1 h, TLC and HPLCanalysis indicated only traces of starting material remaining. Thereaction mixture was diluted with water (79 mL) and the pH was found tobe ˜8.0. The reaction mixture was transferred to a separatory funnel(250 mL) and extracted with ethyl acetate (79 mL) to remove neutralorganic compounds. The aqueous phase was acidified to pH 2 by additionof 6N hydrochloric acid, resulting in a sticky off-white precipitatethat was stored overnight at 5° C. The suspension was transferred to50-mL centrifuge tubes and centrifuged for about 3 min. The supernatantwas decanted and the pellet was washed with cold water and centrifugedthree times. The pellet was suspended in methanol and transferred to a250 mL round-bottomed flask. The suspension was concentrated byrotoevaporation and then coevaporated with dichloromethane to afford acrude pale yellow solid. The crude product was purified by flashchromatography on 24 goof silica gel using 9:1 dichloromethane-methanolas eluting solvent. A second chromatography column was required andemployed 100 g of silica gel and 9:1 dichloromethane-methanol as elutingsolvent, providing 660 mg (86 %) of a pale yellow solid. MS (ESI-) m/z375 (M−H)⁻.

REFERENCES

-   -   Fernandes-Alnemri et al., PNAS USA, 93: 7464 (1996).    -   Garcia-Calvo et al., Cell Death Diff., 6: 362 (1999).    -   Masuda-Nishimura et al., Letters in Applied Microbio., 30: 130        (2000).    -   Miska et al., Biol. Chem. Hoppe-Sexler, 369: 407 (1985).    -   Miska et al., J. Clin. Chem. Clin. Biochem., 25: 23 (1987).    -   Monsees et al., Anal. Biochem., 221: 329 (1994).    -   Monsees et al., J. Biolum. Chemilum., 10: 213 (1995).    -   Nicholson et al., Nature, 376: 37 (1995).    -   Tewari et al., Cell, 81: 801 (1995).    -   Thornberry et al., Nature, 356: 768 (1992).    -   Thornberry et al., J. Biol. Chem., 272: 17907 (1997).    -   White et al., J. Am. Chem. Soc., 88: 2015 (1966).

All publications, patents and patent applications are incorporatedherein by reference. While in the foregoing specification this inventionhas been described in relation to certain preferred embodiments thereof,and many details have been set forth for purposes of illustration, itwill be apparent to those skilled in the art that the invention issusceptible to additional embodiments and that certain of the detailsdescribed herein may be varied considerably without departing from thebasic principles of the invention.

1. A luminescent assay method to detect one or more caspases,comprising: a) contacting a sample suspected of having one or morecaspases with a mixture comprising luciferase and an amino-modifiedaminoluciferin or a carboxy-terminal protected derivative thereof,wherein the modification is the covalent linkage of a substrate for thecaspase to the amino group of aminoluciferin or the derivative thereofvia a peptide bond, and wherein the caspase cleaves the substrate at thepeptide bond; and b) detecting luminescence in the sample, wherein theluminescent assay is more sensitive than a corresponding assay with aconjugate comprising a fluorophore covalently linked to the substrate ora functional equivalent thereof.
 2. A luminescent assay method to detecta protease that specifically cleaves a substrate comprising aspartate,comprising: a) contacting a sample suspected of having one or moreaspartate-specific proteases with a mixture comprising luciferase and anamino-modified aminoluciferin or a carboxy-terminal protected derivativethereof, wherein the modification is the covalent linkage of a substratecomprising aspartate to the amino group of aminoluciferin or thederivative thereof via a peptide bond, and wherein the protease cleavesthe substrate at the peptide bond; and b) detecting luminescence in thesample, wherein the luminescent assay is more sensitive than acorresponding assay with a conjugate comprising a fluorophore covalentlylinked to the substrate or a functional equivalent thereof.
 3. Themethod of claim 1 or 2 further comprising correlating luminescence withcaspase or aspartate-specific protease concentration or activity.
 4. Themethod of claim 1 or 2 which detects a caspase other than caspase 3 orcaspase
 7. 5. The method of claim 1 or 2 which detects caspase 3 orcaspase
 7. 6. The method of claim 5 which detects at least 0.2microunits of caspase.
 7. The method of claim 1 or 2 wherein thesubstrate comprises X₁-X₂-X₃-D, wherein X₁ is Y, D, L, V, I, A, W, or P;X₂ is V or E; and X₃ is any amino acid.
 8. The method of claim 6 whereinX₃ is A, V, H, I, or T.
 9. The method of claim 1 or 2 wherein thesubstrate comprises DEVD (SEQ ID NO:1).
 10. The method of claim 1 or 2wherein the substrate comprises YVAD (SEQ ID NO:2).
 11. The method ofclaim 1 or 2 wherein the substrate comprises LEHD (SEQ ID NO:3).
 12. Themethod of claim 1 or 2 which is at least 2 times more sensitive than acorresponding assay with a conjugate comprising rhodamine-110 covalentlylinked to the substrate.
 13. The method of claim 1 or 2 wherein thesample is a cell lysate.
 14. The method of claim 13 wherein the cellsare treated with an apoptosis inducing agent prior to lysis.
 15. Themethod of claim 1 or 2 wherein the sample comprises intact cells. 16.The method of claim 15 wherein the cells are treated with an apoptosisinducing agent.
 17. The method of claim 1 or 2 wherein the luciferase isa thermostable luciferase.
 18. The method of claim 1 wherein thecarboxy-terminal protected derivative thereof is a compound of formula(I):

wherein R is a peptide that is a substrate for caspase, which is linkedto the remainder of the compound of formula (I) through its C-terminusforming a peptide (amide) bond; and R′ is H or a suitable carboxyprotecting group, or a suitable salt thereof.
 19. The method of claim 18wherein R′ is (C₁–C₆)alkyl, phenyl or benzyl.
 20. The method of claim 2wherein the carboxy-terminal protected derivative thereof is a compoundof formula (I):

wherein R is a peptide that is linked to the remainder of the compoundof formula (I) through an aspartate group at the C-terminus of thepeptide forming a peptide bond; and R′ is H or a suitable carboxyprotecting group, or a suitable salt thereof.
 21. The method of claim 18or 20 wherein R′ is (C₁–C₆)alkyl.
 22. The method of claim 18 or 20wherein R′is methyl, ethyl, propyl, or tert-butyl.
 23. The method ofclaim 18 or 20 wherein R comprises X₁-X₂-X₃-D, wherein X₁ is Y, D, L, V,I, A, W, or P; X₂ is V or E; and X₃ is any amino acid.
 24. The method ofclaim 23 wherein X₃ is A, V, H, I, or T.
 25. The method of claim 23wherein R comprises DEVD (SEQ ID NO:1).
 26. The method of claim 23wherein R comprises YVAD (SEQ ID NO:2).
 27. The method of claim 23wherein R comprises LEHD (SEQ ID NO:3).